Referring to FIG. 1, a conventional bicycle wheel rim is shown to include an extruded metal rim body 1 having an annular spoke-mounting wall 11 with two annular lateral peripheral edges 110 for mounting a plurality of spokes 10, and spaced left and right annular tire-retaining walls 12 which extend integrally, radially, outwardly and respectively from the lateral peripheral edges 110 of the spoke-mounting wall 11. Each of the tire-retaining walls 12 has an inner surface 120 that faces the other one of the tire-retaining walls 12 to confine a tire receiving space 14 therewith, an outer brake pad contacting surface 121 opposite to the inner surface 120, and an annular end face 122 that is distal to the spoke-mounting wall 11. Each of the tire-retaining walls 12 further has a tire-retaining projection 123 formed at a corner of the end face 122 and the inner surface 120. The tire-retaining projection 123 is adapted to engage a bicycle tire 13 so as to the retain the bicycle tire 13 between the tire-retaining walls 12.
A disadvantage that results from the use of the conventional bicycle wheel rim resides in that the tire 13 will expand laterally and outwardly relative to the rim body 1 by virtue of the weight of the rider, thereby causing the tire 13 to abut frictionally against the end face 122 of the rim body 1. The situation is aggravated during a braking operation, where the braking pads 16 clamp together the tire retaining walls 12 at the brake pad contacting surfaces 121. As such, wearing and eventual rupturing of the tire 13 can easily occur when the conventional bicycle wheel rim is in use.